Chemical coordination of gene expression among bacteria as a function of population density is regulated by a mechanism known as ‘quorum sensing’ (QS). Cell-to-cell communication enables single cell organisms to coordinate their behavior so as to adapt to changing environments, allowing them to compete, as well as coexist, with multicellular organisms. Examples of QS-controlled behaviors include biofilm formation, virulence factor expression, antibiotic production and induction of bioluminescence. These processes are beneficial to a bacterial population only when carried out simultaneously. For example, bioluminescence produced by the marine bacterium Vibrio fischeri is beneficial to a number of organisms that host this species but only if a sufficient number of bacteria synchronize their light emission. While various QS signaling systems have been discovered, more proteins and small molecules involved in QS remain to be described (1-4).
The importance of QS in bacteria and its effect on human health is significant, especially when one considers that the total microbial population in the human adult is estimated to exceed the number of mammalian cells by at least a factor of ten. The gastrointestinal tract alone contains 500-1000 different species presenting great genetic diversity, and since most of these species have not yet been cultured in vitro, this population has barely been characterized. Intra- and interspecies QS may very well aid this commensal population in coordinating important processes, such as maintenance of population size and aiding or preventing pathogenic bacterial colonization (5, 6).
QS is regulated by autoinducers that can be categorized into several classes, depending on shared molecular features (FIGS. 1a, 2-4). More than 70 species of Gram-negative bacteria employ N-acyl homoserine lactones (AHLs) as autoinducers, with differences within this class of QS signals occurring in the length and oxidation state of the acyl side chain. Various AHLs from different species have been shown to play important roles in bacterial infections. An important example is the Gram-negative bacterium, Pseudomonas aeruginosa. This common environmental microorganism is an opportunistic human pathogen, being prominent, for example, in patients suffering from cystic fibrosis (CF), a common and lethal inherited genetic disorder, where patients often die due to impaired lung defense functions. A key factor contributing to the pathogenesis and antibiotic resistance of P. aeruginosa lies in its ability to form a biofilm, a microbially-derived sessile community of cells that attach either to an interface or to each other, inhabit an extracellular polymeric matrix, and exhibit a phenotype distinct from that of planktonic cells with respect to growth, gene expression, and protein production. Although the formation and specific architecture of biofilms are regulated by various QS systems (7), as well as other factors, such as cyclic di-GMP, it has been shown that inhibition of even a single QS regulator can lead to a significant decrease in overall biofilm formation.
The primary QS system in P. aeruginosa is regulated through the synthesis and secretion of 3-oxo-C12-HSL, which, upon reaching a threshold concentration, binds the transcriptional activator LasR. This interaction has been proposed to lead to correct folding, followed by dimerization and binding of the LasR dimer to its target DNA, resulting in gene expression. In addition, several other small molecules have been found to play a role in the regulation of gene expression (e.g. C4-HSL, PQS), although the signaling events initiated by 3-oxo-C12-HSL recognition appear to be at the top of the QS hierarchy (8-10). Due to its medical importance, QS in P. aeruginosa has been extensively studied. One notable breakthrough in this field came with the determination of the crystal structure of LasR bound to its natural ligand (3-oxo-C12-HSL), recently reported by Bottomley et al. (11).
Interfering with QS signaling has been explored in recent years as a novel approach to combat pathogenesis. Several groups have identified compounds showing significant inhibition of QS in P. aeruginosa, although the number of strong inhibitors resulting from such efforts remains low. Examples of moderately potent inhibitors, with their IC50 values, are shown in FIG. 1b. 